Microalgae can range from approximately 1 μm to greater than 200 μm in size. Some microalgae form chains or colonies of multiple cells. The composition of algae includes lipids, carbohydrates, ribonucleic acids and proteins. Algae typically require sunlight, water, carbon dioxide and other nutrients in order to grow. The surfaces of algae include negative charges due to the presence of mannuronic acid, β-L-glucuronic acid, β-D-xylosyl, alginic acid and sulfonated polysaccharide residues. The exact composition of the cell walls varies with algae species and conditions of growth.
Culture of microalgae can be practiced for production of hydrocarbons, synthesis of a source of protein, generation of a number of organic substances, wastewater treatment, solar energy conversion and combinations of the these processes. Nutrient supplements rich in carbon dioxide, nitrogen and phosphorous can significantly increase growth rates of algae. Addition of metal ions which generate metal hydroxides can minimize dispersion forces leading to flocculation. Alternatively, high molecular weight organic polymers can flocculate algae by forming a network of bridges. Addition of flocculants into culture medium in order to induce flocculation is a routine procedure in waste water treatment.
Algae solid-aqueous liquid separation processes include screening, filtration (cake filtration and deep bed filtration), microstrainers, sedimentation, flotation, gravity and centrifugation (fixed wall and rotating wall). Wastewater treatment involves the lowering of the suspended solids to a level acceptable for discharge of the water without causing deleterious effects on the ecology of the discharge area.
Sedimentation is a physical water treatment process used to settle out suspended solids in water under the influence of gravity. For example, a water clarifier can be used in the metal finishing industry to remove metal ions from waste water. Alternatively, sedimentation can be used as a primary stage in modern waste water treatment plant, reducing the content of suspended solids as well as pollutants embedded in the suspended solids. Remaining suspended solids can be reduced by chemical coagulation and flocculation in subsequent steps.